One type of light deflecting spatial light modulator (SLM) is the digital micromirror device (DMD). DMDs are available in several different forms including flexure beam, cantilever beam, and both conventional and hidden hinge torsion beam designs. Each type of DMD includes an array of small minors which move out of a resting position, e.g. rotate or deflect, in response to an electrostatic field produced by an electrical signal, typically called an address signal. The resting position of the mirror is typically parallel to the surface of the device. Light is reflected from the surface of the mirror and as the mirror is moved, the direction of the reflected light is changed. The resting position of the mirror is determined by a beam or spring, often called a hinge, which supports the mirror and which stores energy during mirror movement. This stored energy tends to return the mirror to the resting position when the address voltage is removed or reduced.
Deformable micromirror devices are also referred to as DMDs. The difference between digital micromirror devices and deformable micromirror devices is that digital micromirror devices the operated in a bistable mode, as taught in U.S. Pat. No. 5,061,049, issued Oct. 29, 1991, and entitled "Spatial Light Modulator and Method". Digital operation of the micromirror devices includes the application of a bias voltage that ensures that the mirrors have a maximum rotation in either the "on" or "off" direction regardless of the magnitude of the address voltage. The mirror deflection of deformable micromirror devices is an analog function of the voltage applied to the device. The structure of digital micromirror devices and deformable micromirror devices is very similar, and in some cases identical. The disclosed invention may be used in conjunction with either digital, or deformable micromirror devices.
DMDs are typically used in a dark field projection arrangement and can be used, for example, in HDTV applications where a large array of pixels is necessary for the desired image resolution. In addition to the high resolution capabilities of the DMD, another feature that is very useful in video display applications is the speed at which the mirror can be controlled, or the response time of the device. The short response time allows the present generation of DMDs to be toggled on and off up to 180 thousand times each second. Each deflection cycle stores energy in the DMD beam or spring and mechanically stresses the device structure.
DMDs are part of a larger group of devices known as micromechanical devices. Micromechanical devices include some accelerometers, flow sensors, electrical motors, and flow control devices. These devices are often fabricated by processes known as micromachining. Micromachining involves the removal of unwanted material from either the substrate on which the device is being fabricated, or from one or more layers of material that is deposited during the fabrication of the device. The material is typically removed to allow some part of the completed device to move. For example, material must be removed from a motor to allow a rotor to spin around a stationary shaft. In the case of a DMD, material must be removed from below the DMD mirror to allow the mirror to deflect or rotate.
Sometimes an entire layer, called a sacrificial layer, is used during the fabrication process. For example, DMDs are typically fabricated by depositing a sacrificial layer over the circuitry required to deflect the mirror. Mirrors and their hinges are then built on this spacer layer by depositing and patterning one or more metal layers. The metal layers are typically aluminum or an aluminum alloy and are patterned to define a mirror connected to at least one hinge cap by a hinge. In early forms of DMDs, the sacrificial layer was removed from beneath the mirrors and hinges, leaving a portion of the sacrificial layer to support the hinge caps. The mirrors were suspended by the hinges above the wells formed by removing the sacrificial material.
Recent DMD designs include a hole, or via, formed in the sacrificial layer at the location of each hinge cap prior to depositing the hinge metal. When the hinge metal is deposited on the sacrificial layer, it is also deposited on the walls of the via, creating a topless hollow post structure known as a "spacervia." After the mirrors, hinges and hinge caps are patterned, all of the sacrificial layer is removed leaving only the spacervia to support the hinge caps away from the device substrate. Other types of DMDs, such as the so called "Hidden Hinge" torsion beam device as taught by U.S. Pat. No. 5,083,857, issued Jan. 28, 1992 and entitled "Multi-Level Deformable Mirror Device", use two or more sacrificial layers. The hidden hinge torsion beam DMD uses one set of spacervias to support the hinges above the device substrate and a second set of spacervias to support the mirror above the hinges.
The electrostatic forces used to deflect the mirrors generate mechanical stresses in the supporting hinge and spacervia structures. These stresses can lead to a failure in the supporting structure, ruining the device. There is a need in the art for an improved support structure for DMDs and other micromechanical devices.